Mechanism On The Photocatalytic Oxidation Of NO In Gas Phase Over Pt Or Pd Modified TiO₂ Catalysts

The photocatalytic oxidation using titanium dioxide (TiO2) as photocatalyst has been proved to be a suitable method for oxidizing NO to NO2 in gas phase. In order to oxidize higher concentration of NO, further improvement of the characteristics of the photocatalysts is required. Modification of the TiO2 surface with noble metals has been studied in the present dissertation. Three types of noble metal modified TiO2 with different oxidation states were prepared by wet impregnation method, neutralization method and photodeposition method, respectively. And the catalysts were characterized by in situ XPS, in situ FTIR, TPD, et al. The mechanism on photocatalytic oxidation of NO over noble metal modified TiO2 was investigated. Furthermore, one-step hydrothermal synthesis of Pd-modified TiO2 with high photocatalytic activity for NO oxidation in gas phase was developed.Firstly, various surface platinized TiO2 were prepared by three different preparasion methods and investigated with respect to their behaviors in UV photocatalytic oxidation of NO. From the experimental results, it was found that the lifetime of electrons and holes was found prolonged in the PtOx/TiO2 catalysts.The activity tests showed that the dopants existed as metallic Pt0 and PtClx had little contribution to the photocatalytic oxidation of NO in gas phase. This study has been undertaken to investigate the roles of PtO and PtO2 deposits in photocatalytic oxidation of NO over Pt modified TiO2 catalysts. An in situ XPS study indicated that a portion of PtO2 on the surface of Pt/TiO2 was reduced to PtO under UV irradiation. The migration of electrons to PtO2 particles could separate the electrons and holes, resulting in the improvement of photocatalytic activity. And the depletion of PtO2 by electrons could lead to the deactivation of Pt/TiO2 catalyst. Moreover, PtO particles could be corroded to form Pt2+ions by HNO3, which was one of the products of photocatalytic oxidation of NO. NO would adsorb on Pt2+related sites to form Ptn+-NO nitrosyls, retarding photocatalytic oxidation of NO to NO2.Secondly, Pd-modified TiO2 prepared by thermal impregnation method was used in this study for photocatalytic oxidation of NO in gas phase. It was found that Pd dopant existed as Pd2+in as-prepared photocatalysts. And after photocatalytic oxidation reaction, both Pd0 and Pd4+could be detected. The activity tests showed that the modification of Pd could enhance the photocatalytic activity. And it was concluded that Pd4+could be attributed to the improvement of activity. During an in situ XPS study, it was found that the content of hydroxyl groups on the surface of Pd/TiO2 increased when the catalyst was irradiated by UV light, which could result in the improvement of photocatalytic activity. The activity test showed that the optimum Pd dopant content was 0.05%. The relationship between Pd oxidation states on TiO2 photocatalysts and their photocatalytic oxidation behaviors of NO was investigated. Three types of Pd-modified TiO2 with different Pd oxidation states were prepared by wet impregnation method, neutralization method and photodeposition method, respectively. It was found from XPS results that the dominant oxidation states of Pd on these Pd-modified TiO2 catalysts were Pd2+, PdO, and Pd0, respectively. NO-TPD results showed that the NO adsorption capacity was improved greatly by the modification of Pd2+ions. The activity tests showed that Pd-modified TiO2 by a wet impregnation method had a great increase in photocatalytic activity compared to pure TiO2 (Degussa P25). It was concluded that Pd2+ions on as-prepared TiO2 catalysts gave the key contribution to the improvement of photocatalytic activity. However, Pd0 and PdO deposits on TiO2 almost had no positive effect on NO oxidation. The mechanism of photocatalytic oxidation of NO in gas phase over Pd-modified TiO2 was also proposed.Finally, a novel hydrothermal synthesis of Pd-modified TiO2 was developed. The characterizations for the physicochemical properties of the catalysts prepared under different conditions were investigated by XRD, XPS, PL and TEM methods. It was found that the catalyst synthesized by the method performed well on the photocatalytic activity of NO. It was found that the Pt2+doped TiO2 was attributed to the lengthened lifetime of electron-hole pairs.